This invention is in the field of ultrasonic devices and particularly ultrasonic devices which are used in medical practice for providing an image output of various internal structures such as the heart. Systems for performing this function may be found in, for example, co-pending application Ser. No. 173,874 filed July 30, 1980, in the name of James M. Gessert and assigned to the assignee of the present invention and in patents such as Hutchison et al. U.S. Pat. No. 4,149,420 issued Apr. 17, 1979, and Eggleton et al. U.S. Pat. No. 4,234,937 issued Nov. 18, 1980.
In such systems, ultrasonic energy is transmitted into the body by a transducer which, for example, may be pivotly mounted such as is shown in the Donald A. Everet U.S. Pat. No. 4,316,271 issued Feb. 16, 1982, and assigned to the assignee of the present invention. Echos from parts of the body are reflected to the transducer and then converted into electrical signals for use in the imaging systems. The ultrasonic transducer may comprise a piston like element which alternately acts as a transmitter and a receiver producing a pulse of ultrasonic energy and then waiting for the return echo to produce an electric signal, or maybe an array of elements such as the Linear Array Ultrasonic Transducer seen in the George Kossoff U.S. Pat. No. 3,936,791 issued Feb. 3, 1976, which is capable of producing a focused beam of ultrasonic energy. A useful alternate configuration for an array of transducers, particularly in medical field is for the elements to be annularly arranged as concentric rings about a central portion like that shown in the Specht et al. U.S. Pat. No. 4,241,611 issued Dec. 30, 1980.
A difficulty arises when an array of transducing elements is employed by virtue of the fact that the echo returning from a reflecting object produces a wave front which may strike the various elements of the array at different times. If the outputs of the elements are all summed, then at certain positions of the remote object, the delay between the time that the wave front strikes one detector and another may be approximately equal to the wavelength of the energy thus producing a cancellation effect in the summer and a consequent zero output. To overcome this problem, it is common for delay circuits to be employed connected to the various elements so that the signals received by the elements are delayed by various amounts thereby causing the cancellation effect to be overcome and the output of the summer truly additive of the signals received by each of the elements. Delay lines of this sort are shown, for example, in the above referred to U.S. Pat. No. 3,936,791. Ideally, the amount of delay should be altered for each specific distance to the object so that the summed signals remain in phase throughout the entire range of the transducer. Unfortunately, electrically alterable delay lines are difficult and expensive to manufacture and accordingly in practice the delay lines normally have tabs which are switched at appropriate times after a transmit pulse has been sent to provide for several "zones" of delay.